Uncertainties in the purchase price, selling price, and availability of various petrochemical feedstocks and products have plagued the refining industry. One particular area of volatility and uncertainty is in the processing of C.sub.3 and C.sub.4 hydrocarbon feedstocks. C.sub.3 hydrocarbons can be used as fuels with little processing or refined and separated to recover more valuable products such as propylene. The conversion of C.sub.3 products to propylene is usually preferred, however, variations in the price for propylene and the cost of large stand-alone conversion units have discouraged refiners from investment in large scale facilities dedicated to the production of propylene alone. C.sub.4 hydrocarbons find widespread use as direct additives to motor fuel due to their relatively high octane and relatively low price. However, the direct addition of C.sub.4 hydrocarbons is limited by gasoline vapor pressure considerations which may be restricted further in the near future by pending government legislation. These considerations affect the processing of field butanes containing a relatively high amount of normal butanes making it more economically desirable to convert these to alkylate.
C.sub.4 hydrocarbons and C.sub.3 hydrocarbons may be converted to higher molecular weight and higher octane branched paraffins by alkylation. A typical alkylation zone can react butenes or porpylene with isobutane to obtain highly branched C.sub.7 or C.sub.8 isomers. C.sub.4 hydrocarbons are also commonly used to synthesize additives such as MTBE that have low vapor pressure and high octane. In light of these uncertainties and a variation in supply and demand for feedstocks and products, it would be highly desirable to have a process for converting C.sub.3 and C.sub.4 hydrocarbons that provide propylene product or alkylate blending components in varying proportions while at the same time facilitating the operation of the alkylation process and the production of propylene. The ability to produce MTBE as another alternative product would be an added benefit of such a process.
As the value of product propylene by itself rises and regulations restrict the direct blending of butane in motor fuels, it is also highly useful to have a process that can recover increased quantities of propylene directly as product and can also process additional butanes into higher molecular weight and relatively low vapor pressure motor fuel components. It would also be highly beneficial that the process can be integrated into existing alkylation facilities.